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From: "Thomas Magma"
References: <email@example.com> <firstname.lastname@example.org>
Subject: Re: DSP 2's complement vs straight binary
X-Newsreader: Microsoft Outlook Express 5.50.4133.2400
Date: Fri, 18 Oct 2002 04:17:30 GMT
NNTP-Posting-Date: Thu, 17 Oct 2002 22:17:30 MDT
Organization: Shaw Residential Internet
Ya, I don't know what Analog is trying to say there but I'm sure that
inverting the MSB does not convert straight binary to 2's complement. I left
my books at works but here is what I could quickly find on the net to prove
"John Larkin" wrote in
> On Fri, 18 Oct 2002 03:07:07 GMT, "Thomas Magma"
> >If you were to simply invert the MSB it would convert the binary number
> >something called a "sign magnitude" and not to 2's complement. 2's
> >complement is how higher level programming languages treats its 'byte'
> >literals, this what I need.
> >"John Larkin" wrote in
> >message news:email@example.com...
> >> On Fri, 18 Oct 2002 01:08:36 GMT, "Thomas Magma"
> >> wrote:
> >> >Hello,
> >> >
> >> >I am involved in a project where we sub-sample the IF, store it to
> >> >and then post-process it. We are at the stage of selecting a ADC for
> >> >project. I have noticed that a lot of the ADC's suitable for this
> >> >only output straight binary. All of my algorithms to date have used
> >> >complement (signed) values to perform DFT or FFT functions. Processing
> >> >is a major concern and we don't have time to convert each binary
> >> >2's complement. Is there an equivalent method of the DFT or FFT that
> >> >straight binary? Or does anybody know of a 3V ADC that puts out 2's
> >> >compliment and can sub-sample 10.7mhz at less than 1mhz (pipeline)?
> >> >
> >> >Thanks.
> >> >
> >> You can't just invert the MSB?
> >> John
> No, really. Quote from an Analog Devices datasheet:
> Digital Outputs
> The AD9221/AD9223/AD9220 output data is presented in
> positive true straight binary for all input ranges. Table IV indi-
> cates the output data formats for various input ranges regardless
> of the selected input range. A twos complement output data
> format can be created by inverting the MSB.
> So you could do this with one XOR instruction, or just stick a
> hardware inverter in the MSB data line.
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